1. Field of the Invention
The invention relates generally to fixturing arrangements and more particularly to fixturing arrangements for releasably mounting metal components to be treated in a media based finishing process. The invention also relates to methods of finishing engineering components in particular batch finishing where a large number of components are finished together.
2. Description of the Related Art
Procedures for producing a smooth surface finish on metallic components are generally well known. Such procedures include barrel tumbling, abrasive vibratory finishing, grinding, honing, abrasive machining and lapping. Examples of mechanical parts that may be finished using these procedures include splines, crankshafts, camshafts, bearings, gears, continuous velocity (CV) joints, couplings, and journals. Various advantages may be achieved by such finishing including a reduction in wear, friction, noise, vibration, contact fatigue, bending fatigue and operating temperature in the mechanism to which they relate.
The present invention is concerned with media based processes in which a workpiece is acted upon by freely moving particles or granules of an abrasive or non-abrasive medium or by a fluid. These processes may be distinguished from machining processes in which a machining tool acts on the surface of the workpiece.
In the automotive industry, there is an ongoing desire to improve efficiency and performance in order to produce more environmentally friendly vehicles. One area of interest lies in improving the surface finish of certain critical components. At present, a major contributor to drive train friction is the rear axle ring and pinion gearset. Media based finishing processes have been identified as a suitable way to achieve the required degree of smoothness for such components. This is discussed in a number of publications including US2005/0202921A1. As yet however such processes are not able to achieve the production volumes required by the industry in a cost-effective manner.
Various media based techniques are available. Early vibratory finishing techniques used motor-driven vibratory bowls or tubs in which the component would be free floated and allowed to agitate in the presence of abrasive media. By free floated, it is meant the components are allowed to be carried around the vessel by the movement of the media mass. The degree and rate of finishing is primarily controlled by the coarseness, amount and or replenishment of the abrasive grit used in the media mass. Such processes are based on the mass finishing techniques used, for example, for polishing stainless steel tool handles in which ever finer polishing media is used to achieve the desired degree of finish. These procedures are generally recognized as being, too slow for use on hardened engineering components such as case-carburized gearsets.
Further procedures have been developed in which increased levels of mechanical energy are imparted onto the component by moving the component through relatively stationary media. One such procedure is known as drag finishing and is described in e.g. U.S. Pat. No. 4,446,656 to Kobayashi, Another drag finishing machine is described in U.S. Pat. No. 6,918,818 to Böhm, the contents of which are also incorporated herein by reference in their entirety. In this device, individual components may be fixtured to a drive spindle for finishing. The total throughput of components is determined by the process time and the fixturing time for connecting and disconnecting components from the drag spindle.
Another procedure that can achieve an ultra-smooth superfinished surface is chemically accelerated vibratory finishing (CAVF). A chemically accelerated vibratory finishing technique has been developed and described in numerous publications by REM Chemicals, Inc. This technique may be used to refine metal parts to a smooth and shiny surface and has been used commercially for many years. U.S. Pat. No. 4,818,333 to Michaud and U.S. Pat. No. 7,005,080 to Holland, the contents of which are incorporated herein by reference in their entirety, disclose this improved finishing technique. The use of such processes in the automotive industry has been described in an article by Winkelmann, L., J. Holland and R. Nanning in AGMA Paper 04FTM13, entitled “Superfinishing Motor Vehicle Ring and Pinion Gears”. With such a process, a desired finish of around 0.25 micron Ra may be achieved in 30-60 minutes. Such a process is generally unacceptably long for integration into a production line environment.
A number of media based processes can be carried out as batch processes in a bowl or tub on a number of parts at a time. This can reduce the processing time per part but it can unfortunately also lead to components knocking against one another, causing damage and resulting in scrapped parts. Furthermore, it is not convenient for components that have been individually identified or matched. For gear assemblies, it is often the case that two or more components are matched as a gear-set, for example, by a lapping process. Thereafter, it is desirable that the matched parts are kept together during subsequent operations. For such components, batch finishing is generally not suitable as it requires complicated marking and subsequent identification of all components.
In an attempt to overcome the problems of batch processes, vibratory machines have been partitioned with dividers. This is however only practical with a small number of partitions whereby relatively few gear-sets can be finished in each machine. Another problem is that parts are prone to lodge in corners of the bowl, resulting in a non-uniform finish and scrapping of the part. According to GB1018759A, individual containers containing workpieces and finishing medium may be located upon a vibratory machine. The containers are attached to a carrier table by magnets. Such an arrangement of individual containers would not appear practical in keeping large numbers of components separate from one another. It is also doubted that it would be effective on metal components which would themselves be attracted by the magnets.
In conventional machining processes, fixturing devices are used to locate workpieces and prevent them from moving. Various forms of fixturing device are known in the art including clamps, vices, chucks, jigs and the like. For production line manufacturing, any fixturing device must be easy to connect and release, since these procedures will usually add to the overall process time. Fixturing devices of this type are not however commonplace in media based processes and do not appear readily suited for such use. The media used is often relatively small and can become easily lodged in screw threads, clamps and tool openings such as screw and bolt heads. Additionally, the finishing process itself can be detrimental to such fixtures making them difficult to open. For these reasons, conventional mechanical fixtures are not ideally suited for use in media based finishing environments.
A number of magnetic devices are known in which electromagnetic clamps are used which can easily be switched on and off in order to retain and release a workpiece. Examples of such devices are shown in GB847394, EP 1022089 A2 and U.S. Pat. No. 2,913,853. These devices would also not lend themselves to use in a media based finishing environment, where the workpiece is substantially surrounded by and free-floated in the media. Operating with active aqueous chemistry would also pose a serious safety risk. Furthermore, such fixturing devices must be adequately protected from ingress of media or finishing chemistry.
In an alternative device for media based finishing shown in JP57189764A, an electromagnet is used for the purpose of delivering workpieces to a vibratory bowl. The workpieces are guided to move spirally within the bowl by a guide part. Although some reduction in contact damage between components may be achieved, the arrangement does not separately retain matched components and is not suitable for all shapes and size of workpiece.
Thus, there is a particular need for a device and procedure that allows at least some of these problems to be overcome.